The real problem with 4e is that it is just too slow.
I just dont see how blaming the community or lack of decent adventures or lack of DDI is going to solve the real fundamental problem with the rules.
If the game had had a robust VTT--e.g. something like Roll20, but with all powers, feats, etc. pre-coded in just like the CB--that wouldn't have been an issue.
IMO, 4e suffered from five major problems, only one of which was truly system-based.
1. The presentation was bad. It needed another year, and a much more "aesthetic" touch. Even if powers had stayed exactly the same, subtle changes in the graphic design of the books (I'm including "how powers are laid out" in that) would have made it feel more familiar, which people really wanted. The design team took too much to heart the idea that 4e needed to be fresh and new--yes, it needed to be fresh and new
under the hood, but the exterior needed to be instantly recognizable, and it frankly wasn't.
2. The rules were, unfortunately, a little too fiddly for players. While I absolutely loathe 5e turning 99% of bonuses/penalties into Ad/Dis (the end-of-the-line has become the weapon of first resort, and thus the game leaves DMs high and dry for giving further (dis)advantages to players who already have (Dis)Advantage), 4e did go overboard with them. It also went overboard with number of powers, number of feats...it wasn't a LOT overboard, but it was ENOUGH. It's worth noting, though, that a robust VTT would've taken care of all of this by itself, making system fiddly-ness invisible.
3. The digital tools team took a huge blow from
the murder-suicide involving its lead DDI guy, and from what industry people (including Dancey) have said, the team never recovered. The digital tools had already been behind schedule; losing the team lead (and, as far as I can tell, never
properly replacing him) made it impossible to catch up, and was a major factor in WotC eventually dropping the price of the DDI subscription.
4. The adventure-writing was ABSOLUTELY AWFUL. The first and third "introductory" adventures (Keep on the Shadowfell and Pyramid of Shadows) are widely reviled as some of the worst adventures made for 4e, and arguably D&D generally. Rather than published adventures drawing new players in and showing off what the new system could do, they were bland, dry, grindy affairs that front-loaded literally every clunky thing you could find about the system ON TOP OF railroady, terrible narrative design.
5. The initial lack of a new license, and subsequently the EXTREMELY anti-3PP replacement (the GSL), drove any 3PP support 4e might've enjoyed out to the furthest edges, and it took years for non-WotC developers to provide anything. That, coupled with the near-impossibility of bringing non-WotC rules into the official tools, meant WotC directly created a great deal of competition.
Imagine, if you will, an alternate universe. In this universe, the rules of 4e are almost perfectly identical, apart from putting out (say) about 1/3 as much splat content, cutting out all the crappy, rarely-used powers, feats, etc. and somewhat reducing the amount of bonus bloat. Further, extreme caution was taken with the presentation, such that people don't know whether to heap higher praise on the *game* design or the *graphic* design. In this universe, the 2007 economic downturn doesn't cause closures of major bookstores. Unlike our universe, 4e launches with a fully-integrated digital tools suite: a virtual tabletop, monster and character builders, even that silly "character visualizer" (with, naturally, a link to a WotC-owned or -licensed miniature-making company). Said VTT is better than Roll20 is now, and comes pre-built to use unofficial/3rd party rules content if coded in the right form (and with a tool for DMs to write their own powers/feats and package them up as "mods"). WotC keeps the OGL, and instead of cutting ties with Paizo, asks *them* to write the introductory adventures for 4th edition. A little ways down the line, Kingmaker launches as a deluxe digital adventure path, complete with voice acting from well-known figures like Mark Hamill and Ali Hillis (though purely pen-and-paper versions are also produced).
In this alternate universe, it's hard to imagine Pathfinder ever comes into existence, and debatable as to whether Roll20 does either. Especially if WotC had been open to providing its VTT space to non-D&D rulesets (or even just allowing prior editions on it as well as 4e).
I'm not saying even the sum total of this would have made 4e definitely so successful that Hasbro wouldn't still can it for failing to meet Core Brand standards (which may have been impossible even if 110% of everything 4e did was INSANELY successful for a TTRPG). But I am saying that the combination of *relatively subtle* changes to the books themselves (whether in rules or in presentation) plus 4e not having
literally every venture attempting to improve the line (cutting ties with Paizo, dropping the OGL, pushing digital tools) blow up in its face...well, I don't think 5e would be four years old if that were the case. I'm not even sure we'd have 5e at all.